There is described in U.S. Pat. No. 3,772,519, assigned to the same assignee as the present application and specifically incorporated herein by reference, a system for isotope separation, in particular uranium enrichment which produces isotopically selected photoionization of the U.sub.235 isotope in a uranium vapor and acceleration of the ionized particles, by crossed-field magnetohydrodynamic techniques, onto distinct trajectories for separate collection. The photoionization is achieved with laser radiation typically employing the isotopically selective frequency for selective photoexcitation and one or more additional radiations to produce ionization from the excited state.
In order that the efficiency of the isotopically selective photoionization be maintained as high as possible, it is important to control the frequency of at least the laser producing isotopically selective photoexcitation to keep it from deviating from the selected U.sub.235 absorption line and prevent excitation of other undesired isotopes, typically U.sub.238. The higher the spectral accuracy and stability of radiation employed for photoexcitation, the greater is the number of U.sub.235 particles which are photoexcited and the smaller the number of other isotope type particles which are photoexcited.
While laser frequency stabilizers per se are known as, for example, shown in U.S. Pat. No. 3,740,664, special requirements exist in the isotope separation applications. The absorption lines for the U.sub.235 and U.sub.238 isotopes are typically separated by a fraction of a wave number in many cases. Additionally, relatively high photon densities are desirable in the laser radiation for higher enrichment yield, placing a further demand upon the laser system.